Photochromic glass has been widely used in the field of ophthalmic lenses with a particular emphasis on applications to sunglasses. Upon exposure to actinic radiation, a photochromic glass undergoes a photochemical or photostructural transformation that leads to darkening and a reduction in the transmission of light in a particular spectral range. In the case of sunglasses, the actinic radiation may be sunlight and the photochromic response may lead to a reduction in transmission of visible light to reduce the intensity of light that reaches the eye. The photochromic response protects the eye from unsafe intensity levels and provides comfort to the wearer.
The earliest commercially successful photochromic glasses utilized silver halide crystals as the photochromic agent. The silver halides can be incorporated into a variety of base glasses to provide a photochromic glass suitable for ophthalmic uses. Typical base glasses are alkali-doped silica or modified silica glasses. U.S. Pat. No. 3,208,860 (Armistead and Stookey) and U.S. Pat. No. 3,197,296 (Eppler and Stookey), for example, describe photochromic glasses that include silver halides in an alkali-doped boroaluminosilicate glass. Silver halide photochromic glasses remain a viable commercial product and are sold by Corning, Inc. under the PHOTOGRAY® and PHOTOBROWN® product lines.
Silver halides have certain drawback as photochromic agents. First, silver halides have high sensitivity to actinic radiation and impart a strong photodarkening response when exposed to low levels of actinic radiation. As a result, a viewer wearing sunglasses made from silver halide photochromic glass perceives little difference in the level of darkening observed at low (e.g. dawn) and high (e.g. mid-day) levels of illumination. It would be desirable to develop a photochromic glass that exhibits a more uniform photodarkening response, as perceived by the viewer, over the range of illumination intensities normally encountered in daily activity. Second, the photodarkening response of silver halide glasses is sensitive to ambient temperature. For a given level of illumination, photodarkening is more pronounced at low temperatures than at high temperatures. This leads to seasonal variations in the photochromic response that may be undesirable for many consumers. Third, silver is a relative expensive additive for glass and the cost of silver halide photochromic glass limits the range of commercial applications.
Deficiencies in the performance and cost of silver halide glasses have motivated interest in developing silver-free photochromic glasses for commercial applications. U.S. Pat. No. 3,325,299 (Araujo); U.S. Pat. No. 3,954,485 (Seward and Tick); and U.S. Pat. No. 4,166,745 (Araujo and Tick) describe a series of photochromic glasses that use copper-cadmium halides as the photochromic agent. The copper-cadmium halides exhibit good photochromic response and overcome many of the deficiencies associated with silver halides: (1) batch cost is reduced by eliminating silver, (2) the photochromic response is less sensitive to ambient temperature, and (3) a photodarkening response that varies more gradually with illumination intensity. Despite the advantages of the copper-cadmium photochromic glasses, commercial prospects are limited due to concerns over toxicity and disposal of cadmium.
U.S. Pat. No. 3,325,299 (Araujo) also discloses glasses with copper halides as the photochromic agent. The glasses are free of cadmium. The compositions, however, proved difficult to process and exhibited a hazy appearance that was unsuitable for most commercial applications. Haze-free photochromic glass compositions using copper halides as the photochromic agent were presented in U.S. Pat. No. 4,222,781 (Morse and Seward). The base glass composition was a B2O3—Al2O3—SiO2 glass with alkali dopants in addition to copper and halides. Preferred compositions included low concentrations (˜1 wt %) of WO3 or MoO3 to improve the photochromic response of the copper halide. Good photochromic response (darkening and fading) was observed for the glasses.
As the market for ophthalmic glasses has expanded, greater demands have been placed on the performance of photochromic glass. One concern is the recognition that overexposure of the eye to UV light is harmful. This concern has motivated interest in developing photochromic glasses that effectively filter UV light without compromising transmission in the visible. The silver halide, copper-cadmium halide, and copper halide photochromic glasses discussed above have absorption bands in the UV, but the absorption bands lack a sharp UV cutoff and the glasses transmit a significant amount of UV light. U.S. Pat. No. 5,281,562 (Araujo and Morgan) discloses copper halide glasses having sharp UV cutoff near 400 nm. The patent demonstrated that the shape and intensity of UV absorption bands depends on the concentration of copper in the glass. A series of compositions with sharp UV cutoffs were disclosed. The glasses described in U.S. Pat. No. 5,281,562 (Araujo), however, are not photochromic and lack the reversible photodarkening response desired for ophthalmic and other consumer applications.
There is a need for glasses having a sharp UV cutoff and a photochromic response.